1. Field of the Invention
The present invention relates to a rotational speed controller that controls a motor speed depending on a load imposed on a motor.
2. Description of the Related Art
Heretofore, electrically powered tools use a rotational speed control system that automatically reduces a motor rotational speed to a predetermined idling speed when no load is imposed on a motor and automatically increases the motor rotational speed when a load is imposed on the motor, as disclosed in Japanese Patent Application Publication No. 60-77694 and Japanese Patent No. 3301533.
The prior art described above adopts a control system that prevents the motor from reaching a work speed immediately after the tool is powered and also prevents the motor from switching from the work speed to an idling speed when a load is not momentarily imposed on the motor. However, with such a control system, when the load current increases and exceeds a reference value, the motor is switched from the idling speed to the work speed. On the other hand, when the load current decreases and falls below the reference value, the motor is switched from the work speed to the idling speed.
As such, the load current that flows in the motors in saber saws, circular saws and other tools increases immediately after the blade is brought into contact with a workpiece. When the load current exceeds the reference value, the rotational speed of the motor is abruptly switched from the idling speed to the work speed. This abrupt change of the motor speed is dangerous for the operator cutting the workpiece with the saber saw or the like.